KR101631776B1 - Lithium ion secondary battery - Google Patents

Abstract

The negative electrode sheet 240A of the lithium ion secondary battery 100A has a negative electrode current collector 241A and a negative electrode active material layer 243A formed on the negative electrode current collector 241A. The negative electrode active material layer 243A includes scaly graphite particles 710A and includes a first region A1 in the vicinity of the negative electrode collector 241A and a second region A2 in the vicinity of the surface side. 710A are different from each other in the vertical views N1, N2. The vertical degrees N1 and N2 of the graphite particles 710A specify the inclination? N with respect to the surface of the negative electrode collector 241A with respect to the graphite particles 710A and the inclination? N is 60 °? (M1 / m2) where m1 is the number of graphite particles 710A having an angle of 90 deg., And m2 is the number of graphite particles 710A having a gradient? N of 0 deg.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a lithium ion secondary battery,

The present invention relates to a lithium ion secondary battery. In the present specification, the term " secondary battery " The term " lithium ion secondary battery " in this specification refers to a secondary battery in which lithium ions are used as electrolyte ions and charging and discharging are realized by transfer of electrons along with lithium ions in the gaps between the electrodes.

With respect to such a lithium ion secondary battery, for example, Japanese Patent Application Laid-Open No. 2003-197189 discloses a method of manufacturing a negative electrode of a lithium ion secondary battery. Here, in the production method of the negative electrode, a paste in which graphite powder and a binder are dispersed in a solvent is applied to a substrate. Then, the (002) planes of the graphite particles contained in the graphite powder are oriented in the same direction in the magnetic field. Then, in this state, the solvent is removed and the graphite powder is solidified and molded into a binder.

According to such a method for producing a lithium ion secondary battery, the (002) planes of the graphite particles contained in the negative electrode are aligned in the same direction between the graphite particles, and the positive electrode can be arranged in the direction of the (002) plane . This means that lithium ions flowing between the negative electrode and the positive electrode can smoothly penetrate between the layers from the edge portion of the graphite layer upon charging.

Similarly, for the negative electrode for a lithium ion secondary battery, orientation of the (002) plane of the graphite particles to the negative electrode current collector by orienting the graphite particles in the magnetic field is disclosed, for example, in Japanese Patent Application Laid-Open No. 2003-197182, Open Publication No. 2004-220926 and Japanese Patent Application Laid-Open No. 09-245770.

Japanese Patent Application Laid-Open No. 2003-197189 Japanese Patent Application Laid-Open No. 2003-197182 Japanese Patent Application Laid-Open No. 2004-220926 Japanese Patent Application Laid-Open No. 09-245770

However, the lithium ion secondary battery has been mounted on a vehicle as a power source for a mechanism for driving an automobile such as a hybrid vehicle, a plug-in hybrid vehicle, and so-called electric vehicles. In applications such as a vehicle drive battery, for example, in order to improve energy efficiency, it is required to lower the reaction resistance during charging and discharging. In addition, when the automobile is rapidly accelerated, the amount of discharge rapidly increases. Therefore, it is preferable that the discharge resistance at the high rate is suppressed to be low. From this point of view, the direct current resistance of the lithium ion secondary battery tends to be poor only by aligning the (002) plane of the graphite particles with respect to the negative electrode collector by magnetic field orientation with respect to the negative electrode for the lithium ion secondary battery.

The lithium ion secondary battery according to the present invention has a negative electrode current collector and a negative electrode active material layer formed on the negative electrode current collector. The negative electrode active material layer includes scaly graphite particles and the graphite particles have different vertical degrees in the first region near the negative electrode current collector and the second region near the surface side.

In this case, for example, the first region is a region of 0% to 30% in thickness from the negative electrode current collector in the negative electrode active material layer, and the second region is 70% to 100% As shown in Fig.

When the number of graphite particles whose slope θn with respect to the surface of the negative electrode current collector is 60 ° ≦ θn ≦ 90 ° is m1 and the slope θn of the negative electrode current collector with respect to the surface is The number of graphite particles having 0 °?? N? 30 ° is set to m2 (m1 / m2).

In this case, for example, a cross-section SEM image of a plurality of cross-sections is prepared for the negative-electrode active material layer formed on the negative-electrode current collector, and a cross-sectional SEM image of a plurality of cross- And the slope? N with respect to the surface of the negative electrode collector may be specified based on the straight line along the longest diameter in the cross section of the extracted graphite particles.

The absolute value of the difference (N2-N1) between the perpendicularity (N1) of the graphite particles in the first region and the perpendicularity (N2) of the graphite particles in the second region may be, for example, 0.2 or more.

The perpendicularity N1 of the graphite particles in the first region may be N1? 1 and the verticality N2 of the graphite particles in the second region may be N2? 1.2. In this case, the vertical degree N2 of the graphite particles in the second region may be N2? 3.0. Thus, the diffusion resistance of the lithium ion secondary battery can be suppressed to a low level.

In this case, the difference N2-N1 between the verticality N2 of the graphite particles in the second region and the verticality N1 of the graphite particles in the first region may be (N2-N1)? 1.4. Further, the difference (N2-N1) may be (N2-N1)? 2.5. As a result, a lithium ion secondary battery capable of suppressing the diffusion resistance to a low level can be obtained more reliably.

Further, the vertical degree N1 of the graphite particles in the first region is N1? 1.2 and the vertical degree N2 of the graphite particles in the second region is N2? 0.9. In this case, the capacity retention rate can be kept high and the rate of increase in resistance can be suppressed to a low level. In this case, the difference (N2-N1) between the verticality N2 of the graphite particles in the second region and the verticality N1 of the graphite particles in the first region may be (N2-N1)? -0.8.

The method for producing such a lithium ion secondary battery includes a step A of preparing a negative electrode mixture obtained by mixing at least scaly graphite particles and a binder in a solvent, a step A coating the negative electrode mixture produced in the step A on the negative electrode collector, And a step B for forming a negative electrode active material layer on the negative electrode current collector. In this case, the step B is a step of applying the negative electrode mixture to the negative electrode current collector, a drying step of drying the negative electrode material mixture applied to the negative electrode current collector, and a magnetic field applying step, wherein the direction of the graphite particles in the applied negative electrode material mixture is It is only necessary to include an aligning step for adjusting. The application step and the drying step are carried out at least twice, and the negative electrode mixture is applied again to the negative electrode current collector, and the alignment step is carried out at least once before the drying step after the application step.

The orientation process may be performed, for example, before the drying process after the last application process, and the graphite particles in the negative electrode mixture to be applied in the last application process may be formed on the negative electrode current collector. Thereby, the negative electrode active material layer having a high degree of verticality of the graphite particles in the second region near the surface side can be formed. In this case, a rolling step of rolling the negative electrode mixture layer formed on the negative electrode collector before the last application step may be provided. Thereby, the negative electrode active material layer having a large difference between the perpendicularity of the graphite particles in the first region in the vicinity of the negative electrode collector and the perpendicularity of the graphite particles in the second region can be formed.

The orientation step may be carried out before the drying step after the first coating step and the graphite particles in the negative electrode material mixture applied in the first coating step may be formed on the negative electrode current collector. As a result, the negative electrode active material layer having a high degree of verticality of the graphite particles in the first region near the negative electrode current collector can be formed. In this case, after the last drying step, a rolling step of rolling the negative electrode mixture layer formed on the negative electrode current collector may be provided. Thereby, the negative electrode active material layer having a large difference between the perpendicularity of the graphite particles in the first region in the vicinity of the negative electrode collector and the perpendicularity of the graphite particles in the second region can be formed.

1 is a diagram showing an example of the structure of a lithium ion secondary battery.
2 is a view showing a wound electrode body of a lithium ion secondary battery.
3 is a cross-sectional view taken along the line III-III in Fig.
4 is a cross-sectional view showing the structure of the positive electrode active material layer.
5 is a cross-sectional view showing the structure of the negative electrode active material layer.
Fig. 6 is a side view showing a welded portion of an electrode terminal and an uncoated portion of the wound electrode body. Fig.
7 is a diagram schematically showing the state of charging the lithium ion secondary battery.
8 is a diagram schematically showing the state of the lithium ion secondary battery at the time of discharging.
9 is a diagram showing a lithium ion secondary battery according to an embodiment of the present invention.
10 is a cross-sectional view showing the structure of a negative electrode active material layer of a lithium ion secondary battery according to an embodiment of the present invention.
Fig. 11 is a view showing a method of taking a cross-section when obtaining a cross-section SEM image.
Fig. 12 schematically shows a cross section of the extracted graphite particles. Fig.
13 is a cross-sectional view showing another form of the negative electrode active material layer 243A.
Figure 14 is a typical illustration of a Cole-Cole plot (Nyquist plot).
15 is a plot of data of Table 1 for each sample.
16 is a plot of data of Table 2 for each sample.
17 is a plot of data of Table 3 for each sample.
18 is a diagram showing a vehicle equipped with a secondary battery.

First, an example of the structure of a lithium ion secondary battery will be described. Thereafter, a lithium ion secondary battery according to an embodiment of the present invention will be described with appropriate reference to this structural example. In addition, members or portions that exert the same function are appropriately given the same reference numerals. In addition, each drawing is schematically drawn and does not always reflect the real thing. Each drawing shows only one example, and the present invention is not limited unless specifically stated.

Fig. 1 shows a lithium ion secondary battery 100. Fig. 1, the lithium ion secondary battery 100 includes a wound electrode body 200 and a battery case 300. Fig. 2 is a view showing the wound electrode body 200. Fig. Fig. 3 shows a section taken along the line III-III in Fig.

The wound electrode body 200 has a positive electrode sheet 220, a negative electrode sheet 240, and separators 262 and 264 as shown in Fig. The positive electrode sheet 220, the negative electrode sheet 240, and the separators 262 and 264 are belt-like sheet members, respectively.

&Quot; Positive electrode sheet 220 "

The positive electrode sheet 220 includes a strip-shaped positive electrode current collector 221 and a positive electrode active material layer 223. As the positive electrode current collector 221, a metal foil suitable for the positive electrode can be suitably used. As the positive electrode current collector 221, for example, a strip-shaped aluminum foil having a predetermined width and a thickness of approximately 15 mu m can be used. An uncoated portion 222 is set along the rim of the positive electrode current collector 221 on one side in the width direction. 3, the positive electrode active material layer 223 is held on both surfaces of the positive electrode current collector 221 except the unapplied portion 222 set on the positive electrode collector 221 . The positive electrode active material layer 223 contains a positive electrode active material. The positive electrode active material layer 223 is formed by applying a positive electrode mixture containing a positive electrode active material to the positive electrode collector 221.

The " positive electrode active material layer 223 and positive electrode active material particle 610 &

Here, FIG. 4 is a sectional view of the positive electrode sheet 220. 4, the positive electrode active material particles 610, the conductive material 620, and the binder 630 in the positive electrode active material layer 223 are schematically shown schematically so as to clarify the structure of the positive electrode active material layer 223 have. The positive electrode active material layer 223 includes positive electrode active material particles 610, a conductive material 620, and a binder 630 as shown in FIG.

As the positive electrode active material particles 610, a material usable as a positive electrode active material of a lithium ion secondary battery can be used. Examples of the positive electrode active material particles (610), LiNiCoMnO ₂ (lithium-nickel-cobalt-manganese composite oxide), LiNiO ₂ (lithium nickel oxide), LiCoO ₂ (lithium cobaltate), LiMn ₂ O ₄ (lithium manganate), LiFePO ₄ (Lithium iron phosphate), and other lithium transition metal oxides. Here, LiMn ₂ O ₄ has, for example, a spinel structure. LiNiO ₂ or LiCoO ₂ has a layered rock salt structure. LiFePO ₄ has, for example, an olivine structure. In the olivine structure of LiFePO _4, there are particles of, for example, nanometer order. The olivine structure of LiFePO ₄ can also be coated with a carbon film.

&Quot; Conductive material 620 "

Examples of the conductive material 620 include carbon materials such as carbon powder and carbon fiber. As the conductive material 620, one kind selected from these conductive materials may be used alone, or two or more kinds may be used in combination. As the carbon powder, carbon powder such as various kinds of carbon black (for example, acetylene black, oil furnace black, graphitized carbon black, carbon black, graphite, Ketjen black) and graphite powder can be used.

&Quot; Binder 630 "

The binder 630 binds each particle of the positive electrode active material particle 610 and the conductive material 620 contained in the positive electrode active material layer 223 or binds the particle and the positive electrode collector 221 together . As such a binder 630, a polymer soluble or dispersible in a solvent to be used may be used. For example, in a positive electrode material mixture composition using an aqueous solvent, a cellulose polymer (such as carboxymethyl cellulose (CMC), hydroxypropylmethyl cellulose (HPMC), etc.), a fluorine resin (such as polyvinyl alcohol (PVA) (Vinyl acetate copolymer, styrene butadiene copolymer (SBR), acrylic acid modified SBR resin (SBR type latex), etc.), polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer Etc.) can be preferably employed as the water-soluble or water-dispersible polymer. In the positive electrode material mixture composition using a nonaqueous solvent, a polymer (polyvinylidene fluoride (PVDF), polyvinylidene chloride (PVDC), polyacrylonitrile (PAN) or the like) can be preferably employed.

«Thickener, solvent»

The positive electrode active material layer 223 is obtained by preparing a positive electrode material mixture in which the above-described positive electrode active material particles 610 and the conductive material 620 are mixed with a solvent in a paste state (slurry state) Applying, drying and rolling. At this time, both the aqueous solvent and the non-aqueous solvent can be used as the solvent of the positive electrode material mixture. A suitable example of the non-aqueous solvent is N-methyl-2-pyrrolidone (NMP). The polymer material exemplified as the binder 630 may be used for the purpose of exerting a function as a thickener for the positive electrode mixture and other additives in addition to the function as a binder.

The mass ratio of the positive electrode active material to the positive electrode mixture is preferably at least about 50 wt% (typically 50 to 95 wt%), more preferably about 70 to 95 wt% (for example, 75 to 90 wt%) Do. The proportion of the conductive material in the whole positive electrode mixture may be, for example, about 2 to 20 wt%, and usually about 2 to 15 wt%. In the composition using the binder, the proportion of the binder in the whole positive electrode mixture may be, for example, about 1 to 10 wt%, and usually about 2 to 5 wt%.

&Quot; Negative electrode sheet 240 "

As shown in Fig. 2, the negative electrode sheet 240 includes a strip-shaped negative electrode collector 241 and a negative electrode active material layer 243. As the negative electrode current collector 241, a metal foil suitable for the negative electrode may be suitably used. In this negative electrode current collector 241, a strip-shaped copper foil having a predetermined width and a thickness of approximately 10 mu m is used. An uncoated portion 242 is set along the rim portion on the side of the width direction side of the negative electrode collector 241. The negative electrode active material layer 243 is formed on both surfaces of the negative electrode collector 241 except for the unapplied portion 242 set on the negative electrode collector 241. The negative electrode active material layer 243 is held by the negative electrode collector 241 and includes at least a negative electrode active material. The negative electrode active material layer 243 is applied to the negative electrode collector 241 by coating the negative electrode mixture containing the negative electrode active material.

&Quot; Negative electrode active material layer 243 "

5 is a sectional view of the negative electrode sheet 240 of the lithium ion secondary battery 100. FIG. The negative electrode active material layer 243 includes negative electrode active material particles 710, a thickener (not shown), a binder 730, and the like as shown in FIG. 5 schematically shows the negative electrode active material particles 710 and the binder 730 in the negative electrode active material layer 243 so that the structure of the negative electrode active material layer 243 becomes clear.

&Quot; Negative electrode active material particle (710) "

As the negative electrode active material particles 710, one kind or two or more kinds of materials conventionally used for a lithium ion secondary battery can be used without particular limitation. For example, particulate carbon material (carbon particles) containing a graphite structure (layered structure) can be exemplified at least in part. More specifically, the negative electrode active material includes, for example, natural graphite, natural graphite coated with an amorphous carbon material, graphite (graphite), non-graphitized carbonaceous material (hard carbon), graphitized carbonaceous material (soft carbon) Or a combination of these materials. Here, the negative electrode active material particle 710 shows a case where so-called scaly graphite is used, but the negative electrode active material particle 710 is not limited to the illustrated example.

«Thickener, solvent»

The negative electrode active material layer 243 is obtained by preparing a negative electrode mixture obtained by mixing the above-described negative electrode active material particles 710 and the binder 730 in a paste state (slurry state) with a solvent and applying the negative electrode active material layer 710 to the negative electrode collector 241 Followed by drying and rolling. At this time, both of an aqueous solvent and a non-aqueous solvent can be used as the solvent of the negative electrode mixture. A suitable example of the non-aqueous solvent is N-methyl-2-pyrrolidone (NMP). As the binder 730, a polymer material exemplified as the binder 630 of the positive electrode active material layer 223 (see FIG. 4) can be used. The polymer material exemplified as the binder 630 of the positive electrode active material layer 223 may be used for the purpose of exerting a function as a thickener for the positive electrode mixture and other additives in addition to the function as a binder.

&Quot; Separators 262 and 264 "

The separators 262 and 264 are members that isolate the positive electrode sheet 220 and the negative electrode sheet 240 as shown in FIG. 1 or FIG. In this example, the separators 262 and 264 are formed of a sheet-shaped sheet material having a plurality of minute holes and having a predetermined width. As the separators 262 and 264, for example, a separator having a single-layer structure made of a porous polyolefin-based resin or a separator having a laminated structure can be used. In this example, the width b1 of the negative electrode active material layer 243 is slightly wider than the width a1 of the positive electrode active material layer 223, as shown in Figs. 2 and 3. The widths c1 and c2 of the separators 262 and 264 are slightly larger than the width b1 of the negative electrode active material layer 243 (c1, c2> b1> a1).

In the examples shown in Figs. 1 and 2, the separators 262 and 264 are formed of sheet-shaped members. The separators 262 and 264 may be a member that insulates the positive electrode active material layer 223 and the negative electrode active material layer 243 from each other and allows the movement of the electrolyte. Therefore, it is not limited to a sheet-like member. The separators 262 and 264 may be formed of a layer of insulating particles formed on the surface of the positive electrode active material layer 223 or the negative electrode active material layer 243 instead of the sheet- Here, examples of the particles having an insulating property include an inorganic filler having an insulating property (for example, a filler such as a metal oxide or a metal hydroxide) or a resin particle having insulating properties (for example, particles such as polyethylene or polypropylene) You can.

&Quot; Battery case (300) "

In this example, as shown in Fig. 1, the battery case 300 is a so-called rectangular battery case, and includes a container body 320 and a lid 340. As shown in Fig. The container main body 320 is a flat box-shaped container having a bottomed square shape and one side (upper surface) thereof being opened. The lid 340 is a member provided in the opening (upper opening) of the container body 320 and blocking the opening.

BACKGROUND ART In a vehicle-mounted secondary battery, it is desired to improve weight energy efficiency (capacity of a battery per unit weight) in order to improve fuel economy of a vehicle. For this reason, in this embodiment, the container body 320 and the lid 340 constituting the battery case 300 are made of a light metal such as aluminum or aluminum alloy. As a result, the weight energy efficiency can be improved.

The battery case 300 has a flat rectangular internal space as a space for accommodating the wound electrode body 200. [ 1, the flat inner space of the battery case 300 is slightly wider than the wound electrode body 200 in width. In this embodiment, the battery case 300 includes a container body 320 having a bottomed square shape and a lid 340 covering the opening of the container body 320. In addition, electrode terminals 420 and 440 are provided on the cover 340 of the battery case 300. The electrode terminals 420 and 440 pass through the battery case 300 (the lid 340) and extend out of the battery case 300. In addition, the lid 340 is provided with a liquid injection hole 350 and a safety valve 360.

As shown in Fig. 2, the wound electrode body 200 is pressed flat in one direction orthogonal to the winding axis WL. 2, the uncoated portions 222 of the positive electrode current collector 221 and the uncoated portions 242 of the negative electrode collector 241 are formed in a spiral shape on both sides of the separators 262 and 264 . 6, the middle portions 224 and 244 of the uncoated portions 222 and 242 are gathered together and welded to the tip portions 420a and 440a of the electrode terminals 420 and 440 in this embodiment have. At this time, for example, ultrasonic welding is used for welding the electrode terminal 420 and the positive electrode current collector 221 from the difference of each material. For welding the electrode terminal 440 and the negative electrode collector 241, for example, resistance welding is used. 6 is a side view showing a welded portion of the intermediate portion 224 (244) of the uncoated portion 222 (242) of the wound electrode body 200 and the electrode terminal 420 (440) 1 is a sectional view taken along line VI-VI in Fig.

The wound electrode body (200) is mounted on the electrode terminals (420, 440) fixed to the lid (340) in a state where the electrode body (200) is pressed in a flat state. The wound electrode body 200 is accommodated in a flat inner space of the container body 320, as shown in Fig. The container body 320 is clogged by the lid 340 after the wound electrode body 200 is housed. The lid 340 and the joint 322 (see Fig. 1) of the container body 320 are welded and sealed by, for example, laser welding. Thus, in this example, the wound electrode body 200 is positioned in the battery case 300 by the electrode terminals 420 and 440 fixed to the lid 340 (the battery case 300).

«Electrolyte»

Thereafter, the electrolytic solution is injected into the battery case 300 from the instilling hole 350 provided in the lid 340. As the electrolytic solution, a so-called non-aqueous electrolytic solution which does not use water as a solvent is used. In this example, the electrolytic solution is an electrolytic solution containing LiPF ₆ in a concentration of about 1 mol / liter in a mixed solvent of ethylene carbonate and diethyl carbonate (for example, a mixed solvent having a volume ratio of about 1: 1) have. Thereafter, a metallic sealing cap 352 is provided (for example, welded) on the liquid injection hole 350 to seal the battery case 300. Further, the electrolytic solution is not limited to the electrolytic solution exemplified here. For example, a non-aqueous electrolyte used conventionally in a lithium ion secondary battery can be suitably used.

«Public (vacancy)»

Here, the positive electrode active material layer 223 has a minute gap 225, which may also be referred to as a cavity, for example, between the particles of the positive electrode active material particles 610 and the conductive material 620 ). An electrolytic solution (not shown) may permeate into the minute gap of the positive electrode active material layer 223. The negative electrode active material layer 243 has a minute gap 245, which may also be referred to as a cavity, for example, between the particles of the negative electrode active material particles 710 (see FIG. 5). Here, these gaps 225 and 245 (cavities) are appropriately referred to as " holes ". 2, the wound electrode body 200 is wound in the form of a spiral on both sides along the winding axis WL, with the non-coated portions 222 and 242 being wound therearound. The electrolytic solution can permeate from the gap between the uncoated portions 222 and 242 on both sides 252 and 254 along the winding axis WL. Therefore, in the interior of the lithium ion secondary battery 100, the electrolytic solution evenly penetrates the positive electrode active material layer 223 and the negative electrode active material layer 243.

«Gas discharge path»

Further, in this example, the flat internal space of the battery case 300 is slightly wider than the flattened wound electrode body 200. Gaps 310 and 312 are provided on both sides of the wound electrode body 200 between the wound electrode body 200 and the battery case 300. The clearances 310 and 312 become gas discharge paths. When the temperature of the lithium ion secondary battery 100 abnormally increases, for example, when an overcharge occurs, the electrolyte may be decomposed and the gas may be abnormally generated. In this embodiment, the abnormally generated gas moves toward the safety valve 360 through the gaps 310 and 312 between the wound electrode body 200 and the battery case 300 at both sides of the spiral electrode assembly 200 , And exhausted from the safety valve 360 to the outside of the battery case 300.

In this lithium ion secondary battery 100, the positive electrode collector 221 and the negative electrode collector 241 are electrically connected to the external device through the electrode terminals 420 and 440 penetrating the battery case 300 . Hereinafter, the operation of the lithium ion secondary battery 100 during charging and discharging will be described.

«Operation during charging»

Fig. 7 schematically shows the state of charging the lithium ion secondary battery 100 at this time. 7, the electrode terminals 420 and 440 (see Fig. 1) of the lithium ion secondary battery 100 are connected to the charger 290. [ The lithium ion (Li) is discharged from the positive electrode active material in the positive electrode active material layer 223 into the electrolyte solution 280 by the action of the charger 290 during charging. Charge is also released from the positive electrode active material layer 223. The discharged electricity is sent to the positive electrode current collector 221 through a conductive material (not shown), and is also sent to the negative electrode sheet 240 through the charger 290. Charges are accumulated in the negative electrode sheet 240 and lithium ions (Li) in the electrolyte 280 are absorbed by the negative electrode active material in the negative electrode active material layer 243 and are also stored.

≪ Operation at discharge >

Fig. 8 schematically shows the state of the lithium ion secondary cell 100 at the time of discharging. Charge is sent from the negative electrode sheet 240 to the positive electrode sheet 220 and the lithium ions stored in the negative electrode active material layer 243 are discharged to the electrolyte 280 as shown in FIG. In the positive electrode, lithium ions in the electrolyte solution 280 are introduced into the positive electrode active material in the positive electrode active material layer 223.

In this manner, lithium ions come and go between the positive electrode active material layer 223 and the negative electrode active material layer 243 through the electrolyte 280 in the charging and discharging of the lithium ion secondary battery 100. Further, at the time of charging, electric charge is sent from the positive electrode active material to the positive electrode current collector 221 through the conductive material. On the other hand, at the time of discharging, the charge is returned from the positive electrode current collector 221 to the positive electrode active material through the conductive material.

At the time of charging, it can be considered that as the movement of lithium ions and the movement of electrons become smooth, efficient and rapid charging becomes possible. At the time of discharging, it can be considered that as the movement of lithium ions and the movement of electrons are smooth, the resistance of the battery decreases, the amount of discharge increases, and the output of the battery improves.

«Other battery types»

The above shows an example of a lithium ion secondary battery. The lithium ion secondary battery is not limited to the above embodiment. In addition, similarly, the electrode sheet on which the electrode mixture is applied to the metal foil is used in various other battery types. For example, as another type of battery, a cylindrical battery or a laminate type battery is known. The cylindrical battery is a battery in which a wound electrode body is housed in a cylindrical battery case. A laminated battery is a battery in which a positive electrode sheet and a negative electrode sheet are laminated via a separator.

Hereinafter, a lithium ion secondary battery according to an embodiment of the present invention will be described. Since the basic structure of the lithium ion secondary battery to be described herein is the same as that of the lithium ion secondary battery 100 described above, it will be described suitably with reference to the drawings of the lithium ion secondary battery 100 described above.

&Quot; Lithium ion secondary battery (100A) "

Fig. 9 shows a lithium ion secondary battery 100A according to an embodiment of the present invention. 9, in the embodiment of the present invention, the structure of the negative electrode sheet 240A is different from that of the lithium ion secondary battery 100 shown in Fig. An uncoated portion of the negative electrode sheet 240A is indicated by reference numeral 242A. 10 is a sectional view of a negative electrode sheet 240A of a lithium ion secondary battery 100A according to an embodiment of the present invention.

«Graphite particles 710A»

This lithium ion secondary battery 100A has a negative electrode collector 241A and a negative electrode active material layer 243A formed on the negative electrode collector 241A as shown in Fig. As shown in Fig. 10, the negative electrode active material layer 243A is composed of scaly graphite particles (the scaly graphite particles are also referred to as flake graphite) as the negative electrode active material particles 710A, .

The graphite particles 710A have a layer structure in which the carbon hexagonal mesh plane overlaps to form a plurality of layers. In the graphite particles 710A, at the time of charging, the lithium ions penetrate between the layers of the graphite particles 710A from the edge portions (edge portions of the layers) of the graphite particles 710A and spread between the layers of the graphite particles 710A .

In this embodiment, as shown in Fig. 10, such scaly graphite particles 710A include graphite particles at least a part of which is covered with an amorphous carbon film 750. The core of the negative electrode active material particle 710A covered by the amorphous carbon film 750 may be, for example, natural graphite.

<< Amorphous carbon film (750) »

The amorphous carbon film 750 is a film made of an amorphous carbon material. For example, graphite particles that are at least partially covered with the amorphous carbon film 750 can be obtained by mixing and firing graphite particles that become nuclei of the negative electrode active material particles 710A.

Here, the weight ratio X of the amorphous carbon film 750 in the negative electrode active material particle 710A may be approximately 0.01? X? 0.10. The weight ratio X of the amorphous carbon film 750 is more preferably 0.02? X and the upper limit is more preferably X? 0.08 and X? 0.06. As a result, the amorphous carbon film 750 can more appropriately cover the negative electrode active material particles 710A.

&Quot; Negative electrode active material layer 243A &

10, the negative electrode active material layer 243A is formed of a first region A1 in the vicinity of the negative electrode collector 241A and a second region A2 in the vicinity of the surface side of the graphite particles 710A ) Are different from each other.

&Quot; First area (A1) &quot;

Here, the first region A1 is a region in the anode active material layer 243A near the anode current collector 241A. The first region A1 is a region of 0% to 30% in thickness from the negative electrode collector 241A in the negative electrode active material layer 243A, for example. When the thickness of the negative electrode active material layer 243A is about 100 占 퐉, the first area A1 near the negative electrode collector 241A is a region about 30 占 퐉 thick from the surface of the negative electrode collector 241A.

&Quot; Second area A2 &quot;

The second region A2 is a region in the vicinity of the surface side of the negative electrode active material layer 243A. The cell structure is a region in the vicinity of the surface of the negative electrode active material layer 243A facing the separators 262 and 264 (the positive electrode active material layer 223 (see FIG. 9, for example)). The second area A2 is, for example, a region from 70% to 100% in thickness from the negative electrode collector 241A in the negative electrode active material layer 243A. In other words, the second region A2 is a region where the thickness of the negative electrode active material layer 243A is 30% from the surface side of the negative electrode active material layer 243A. Here, the second region A2 is, in another view, a region in the vicinity of the surface opposed to the separators 262 and 264 (the positive electrode active material layer 223 (see FIG. 9)) in the negative electrode active material layer 243A. For example, if the thickness of the negative electrode active material layer 243A is about 100 占 퐉, the second area A2 near the surface side is a thickness of about 30 占 퐉 from the surface of the negative electrode active material layer 243A.

&Quot; Verticality Nx of graphite particles 710A &quot;

Here, the vertical degree Nx of the graphite particles 710A in each region can be obtained as follows.

First, a cross section SEM image of a plurality of cross sections is prepared for the negative electrode active material layer 243A formed on the negative electrode current collector 241A. Next, a predetermined number of graphite particles 710A are extracted from the side having a larger cross-sectional area of the outer tube in the plurality of cross-sectional SEM images. Subsequently, on the basis of a straight line along the longest diameter (the longest distance in the cross section of the extracted graphite particles 710A) in the cross section of the extracted graphite particles 710A, the surface of the negative electrode current collector 241A Theta] n. When the number of graphite particles 710A having a slope? N of 60 占?? N? 90 占 is m1 and the number of graphite particles 710A having a slope? N of 0 占? do. The perpendicularity Nx of the graphite particles 710A was (m1 / m2).

&Quot; Verticality Nx of graphite particles 710A &quot; = (m1 / m2)

Herein, m1 is the number of graphite particles 710A having a degree of inclination? N of 60 degrees? N? 90 degrees with respect to the anode current collector 241A and standing relatively with respect to the anode current collector 241A. M2 is the number of the graphite particles 710A which are inclined relative to the anode current collector 241A with a degree of inclination θ n of 0 ° ≤θn ≤ 30 ° and relatively lie against the anode current collector 241A.

As described above, the vertical degree Nx of the graphite particles 710A is determined by the number of graphite particles 710A relatively standing on the negative electrode current collector 241A / (graphite particles 710A relatively standing on the negative electrode current collector 241A) The number of particles 710A). The vertical degree Nx of the graphite particles 710A can be an index for evaluating to what extent the graphite particles 710A stand with respect to the negative electrode current collector 241A among the negative electrode active material layers 243A. That is, when the vertical degree Nx is 1, it indicates that the number of the graphite particles 710A relatively standing relative to the anode current collector 241A is the same as the number of graphite particles 710A laid relatively. On the other hand, as the vertical degree becomes larger than 1, it can be estimated that the graphite particles 710A stand on the anode current collector 241A. On the other hand, it can be estimated that the graphite particles 710A are laid down with respect to the anode current collector 241A as the vertical degree becomes smaller than 1.

&Quot; Cross-sectional SEM image &

Here, when obtaining the vertical degree Nx of the graphite particles 710A, a cross-sectional SEM image of a plurality of cross-sections is prepared. At this time, for example, when viewed in plan, a plurality of cross sections arranged substantially equally to the negative electrode current collector are set, and a cross-sectional SEM image of the cross section is prepared. By preparing cross-sectional SEM images in a plurality of sections in this way, even when the graphite particles 710A are uniformly directed in a constant direction, the verticality can be appropriately evaluated.

In this case, for example, as shown in Fig. 11, the negative electrode active material layer 243A formed on the negative electrode current collector 241A is formed to have an angle of 0 deg. And 45 deg. , 90 °, and 135 ° are prepared. Here, four cross sections of 0 deg., 45 deg., 90 deg., And 135 deg. Can be obtained as SEM images of cross sections of the negative electrode active material layer 243A cut at substantially a predetermined angle.

In Fig. 11, the intersection points of the respective cross sections coincide with each other, but the intersections of the cross section need not coincide with each other. Although four cross sections uniformly arranged at 45 占 are considered here, six cross sections evenly arranged at approximately 30 占 may be considered. As described above, it is sufficient to prepare a plurality of cross-sections arranged substantially equally to the anode current collector 241A in plan view, and prepare cross-sectional SEM images of the cross-sections.

<< Extraction of Graphite Particles 710A >>

Next, a predetermined number of graphite particles 710A are extracted from the side having a larger cross-sectional area of the outer tube in the plurality of cross-sectional SEM images. In this embodiment, a predetermined number of graphite particles 710A are extracted in the first region A1 and the second region A2, respectively. At this time, a predetermined number of graphite particles 710A are extracted from the graphite particles 710A including at least a part of the first region A1 and the second region A2.

In this embodiment, for example, a cross-sectional SEM image in which 100 or more graphite particles 710A are respectively taken in the first region A1 and the second region A2 may be prepared. Then, a predetermined number of graphite particles 710A (for example, about 30) from the cross-sectional SEM image in the first area A1 and the second area A2, respectively, ).

In this embodiment, the graphite particles 710A are scaly graphite, and are not spherical. In this case, it is highly possible that the graphite particles 710A having a larger cross-sectional area of the outer surface of the cross-sectional SEM image are sections along the longest distance of the graphite particles 710A. Thus, by extracting the graphite particles 710A of about 3% from the side having a larger cross-sectional area of the outer appearance, it is possible to extract the graphite particles 710A close to the cross section along the longest distance.

The inclination? N of the graphite particles 710A

Fig. 12 schematically shows a cross-section of the extracted graphite particles 710A. The gradient? N of the graphite particles 710A with respect to the surface of the negative electrode collector 241A is specified based on the straight line L along the longest distance of the extracted graphite particles 710A.

When the number of graphite particles 710A having a slope? N of 60 占?? N? 90 占 is m1 and the number of graphite particles 710A having a slope? N of 0 占? , And the vertical degree Nx of the graphite particles 710A is Nx = (m1 / m2). The vertical degree Nx of the graphite particles 710A was measured in each of the first region A1 and the second region A2. Here, the graphical representation Nx of the graphite particles 710A in the first region A1 of the negative electrode active material layer 243A is referred to as N1 and the graphite particles 710A of the second region A2 of the negative electrode active material layer 243A And the vertical degree Nx of the particle 710A is referred to as &quot; N2 &quot;.

Here, the negative electrode active material layer 243A is formed of the graphite particles 710A in the first region A1 near the negative electrode collector 241A, The vertical degree N2 of the graphite particles 710A in the second region A2 near the surface side is different.

10, in the first region A1 near the anode current collector 241A, the vertical degree N1 of the graphite particles 710A is small and the second region A2 , The vertical degree N2 of the graphite particles 710A is large. 10, the graphite particles 710A are laid in the first region A1 near the anode current collector 241A and the graphite particles 710A are located in the second region A2 near the front surface side. Standing.

Thus, in this embodiment, the vertical degree Nx of the graphite particles 710A is different between the first region A1 near the anode current collector 241A and the second region A2 near the surface side. In this embodiment, the group of graphite particles 710A having different vertical degrees Nx cross each other in the boundary region between the first region A1 and the second region A2. As a result, the contact between the graphite particles 710A in the boundary region increases. In this case, the difference (N2-N1) between the verticality N1 of the graphite particles 710A of the first region A1 and the verticality N2 of the graphite particles 710A of the second region A2 The value may be, for example, 0.2 or more (| N2-N1 |? 0.2), for example, 0.3 or more (| N2-N1 |? 0.3). Thereby, the effect of making the verticality N1 of the graphite particles 710A of the first region A1 different from the verticality N2 of the graphite particles 710A of the second region A2 can be more reliably .

<< Production Method of Negative Electrode Active Material Layer 243A >>

The negative electrode active material layer 243A includes, for example, a step A for preparing a negative electrode mixture and a step B for forming a negative electrode active material layer 243A in the negative electrode collector 241A. In Process A, a negative electrode mixture prepared by mixing at least graphite particles 710A and binder 730 in a solvent is prepared. In Step B, the negative electrode mixture prepared in Step A is applied to the negative electrode collector 241A.

Specifically, Step B includes a coating step, a drying step and an orientation step. The application step is a step of applying the negative electrode mixture to the negative electrode collector 241A. The drying step is a step of drying the negative electrode material mixture applied to the negative electrode collector 241A. The aligning step is a step of applying a magnetic field and adjusting the direction of the graphite particles 710A in the applied negative electrode mixture. In this embodiment, the application step and the drying step are carried out at least twice, and the negative electrode mixture is applied to the negative electrode collector 241A again. The alignment step may be performed at least once before the drying step after the application step.

In the &quot; second region A2, the graphite particles 710A are in a linear shape &

In this case, the aligning step may be carried out before the drying step after the last coating step, and the graphite particles in the negative electrode material mixture applied in the last coating step may be formed on the negative electrode current collector. 10, the graphite particles 710A are laid in the first region A1 near the anode current collector 241A and the graphite particles 710A are deposited in the second region A2 near the front surface side, May be used to fabricate the linear positive electrode active material layer 243A.

In this case, a rolling step of rolling the negative electrode mixture layer formed on the negative electrode collector 241A before the last application step may be provided. For example, in the case of coating the negative electrode current collector 241A by applying the negative electrode mixture in two portions, the rolling step may be performed, for example, after the first drying step and before the second coating step. This makes it possible to lower the vertical degree N1 of the graphite particles 710A in the first region A1 in the vicinity of the anode current collector 241A. The difference between the vertical degree N1 and the vertical degree N2 of the graphite particles 710A in the second region A2 can be increased.

In the example shown in Fig. 10, graphite particles 710A are laid in the first region A1 near the negative electrode collector 241A and the graphite particles 710A are located in the vicinity of the surface side facing the positive electrode active material layer 223 And graphite particles 710A stand in the second region A2. Therefore, lithium ions easily enter the negative electrode active material layer 243A at the time of charging, and lithium ions are liable to be released from the negative electrode active material layer 243A at the time of discharging. In the example shown in Fig. 10, graphite particles 710A are laid in the first region A1 near the anode current collector 241A. In this case, lithium ions are likely to enter from the surface side of the negative electrode active material layer 243A. The lithium ions entering the negative electrode active material layer 243A tend to diffuse in the first region A1 in the negative electrode active material layer 243A near the negative electrode collector 241A.

10, in the form in which the graphite particles 710A are laid in the first region A1 and the graphite particles 710A are standing in the second region A2, for example, the first region A1 M1 / m2) of the graphite particles 710A of the second region A2 is m1 / m2 and the verticality Nx of the graphite particles 710A of the second region A2 is m1 / m2) is (m1 / m2)? 1.2.

In this case, the perpendicularity Nx (m1 / m2) of the graphite particles 710A in the first region A1 may be (m1 / m2)? 0.8. More preferably, the perpendicularity Nx (m1 / m2) of the graphite particles 710A in the first region A1 may be (m1 / m2)? 0.6.

The perpendicularity Nx (m1 / m2) of the graphite particles 710A in the second region A2 may be (m1 / m2)? 1.5. More preferably, the perpendicularity Nx (m1 / m2) of the graphite particles 710A in the second region A2 may be (m1 / m2)? 2.0. More preferably, the perpendicularity Nx (m1 / m2) of the graphite particles 710A in the second region A2 may be (m1 / m2)? 3.0.

In this case, the difference (N2-N1) between the perpendicularity N2 of the graphite particles 710A in the second region A2 and the perpendicularity N1 of the graphite particles 710A in the first region A1, The diffusion resistance of lithium ions in the lithium ion secondary battery 100A tends to decrease. For example, the difference N2-N1 between the perpendicularity N2 of the graphite particles 710A in the second region A2 and the perpendicularity N1 of the graphite particles 710A in the first region A1 is (N2-N1) &gt; = 1.4. More preferably, the difference (N2-N1) between the perpendicularity N2 of the graphite particles 710A in the second region A2 and the perpendicularity N1 of the graphite particles 710A in the first region A1, (N2-N1) &gt; 2.5. As described above, the difference (N2-N1) between the perpendicularity N2 of the graphite particles 710A of the second region A2 and the perpendicularity N1 of the graphite particles 710A of the first region A1 becomes positive , And the larger the difference is, the more the lithium ion diffusion resistance of the lithium ion secondary battery 100A tends to be lowered remarkably.

&Quot; In the first region A1, the graphite particles 710A have a linear shape &

13 is a sectional view showing another form of the negative electrode active material layer 243A. 13, the graphite particles 710A are in contact with the negative electrode collector 241A in the first area A1 and the graphite particles 710A are discharged from the negative electrode collector 241A ). In other words, the vertical degree Nx of the graphite particles 710A in the first region A1 is large and the vertical degree Nx of the graphite particles 710A in the second region A2 is small.

In this case, the alignment step is carried out after the first coating step and before the drying step, and the graphite particles 710A in the negative electrode mixture mixed in the first coating step may be placed on the negative electrode current collector 241A. In this case, the negative electrode material mixture dried in the first drying step may be subjected to the second or subsequent coating step without rolling, and the negative electrode material mixture dried in the last drying step may be rolled. At this time, the rolling amount can be reduced to such an extent that the graphite particles 710A in the first region A1 near the anode current collector 241A do not lie. As a result, the graphite particles 710A in the second region A2 near the surface side are formed while maintaining the vertical degree Nx of the graphite particles 710A in the first region A1 near the anode current collector 241A, It is possible to reduce the vertical degree Nx of the light emitting diode. As described above, by properly adding the rolling process, the difference in the vertical degree Nx of the graphite particles 710A in the second region A2 near the surface of the first region A1 can be appropriately adjusted.

13, graphite particles 710A are present in the first region A1 near the negative electrode collector 241A and the graphite particles 710A are present in the vicinity of the surface side facing the positive electrode active material layer 223 (see Fig. 9) And the graphite particles 710A are laid in the second region A2. As a result, the negative active material layer 243A has a fast reaction with lithium ions in the first region A1 near the negative electrode collector 241A. As a result, for example, the negative electrode active material layer 243A having a high rate of charging or discharging at high rate can be obtained. Also, since the graphite particles 710A are laid in the second region A2 near the surface side, the lithium ions accumulated in the negative electrode active material layer 243A are hardly released. This makes it easy to maintain the capacity of the lithium ion secondary battery 100A (see FIG. 9) at a high level.

In this case, when the vertical degree N1 of the graphite particles 710A in the first region A1 is N1? 1.2 and the vertical degree N2 of the graphite particles 710A in the second region A2 is N2? 0.9 do. As a preferable form, the difference (N2-N1) between the verticality N2 of the graphite particles 710A in the second region A2 and the verticality N1 of the graphite particles 710A in the first region A1 ) Is (N2-N1)? -0.8. In this case, the difference (N2-N1) between the perpendicularity N2 of the graphite particles 710A in the second region A2 and the perpendicularity N1 of the graphite particles 710A in the first region A1 is negative . The larger the absolute value, the better the capacity retention rate and the rate of resistance increase of the lithium ion secondary battery 100A.

«Evaluation cell»

The present inventor has found that the graphite particles 710A of the first region A1 in the vicinity of the negative electrode collector 241A and the second region A2 in the vicinity of the surface side of the negative electrode sheet 240A, A plurality of samples in which the anode active material layer 243A having a different vertical degree N2 of the graphite particles 710A were prepared. Then, evaluation cells were prepared using each of the negative electrode sheets 240A, and the DC resistance, the diffusion resistance, the capacity retention rate, and the rate of increase in resistance were evaluated. Here, the evaluation cell is an 18650 type battery having a rated capacity of 250 mAh.

&Quot; Positive electrode of evaluation cell &quot;

Here, as the positive electrode of the evaluation cell, an aluminum foil having a thickness of 15 mu m was used for the positive electrode current collector. The solid content of the positive electrode mixture prepared at the time of forming the positive electrode active material layer was set at a weight ratio of the positive electrode active material: conductive material: binder = 87: 10: 3. As the positive electrode active material, particles of LiNiCoMnO ₂ (lithium nickel cobalt manganese composite oxide) are used, and a common positive electrode active material is used in each evaluation cell. Acetylene black is used as a conductive material. Polyvinylidene fluoride (PVDF) is used as a binder.

&Quot; Negative electrode of evaluation cell &

A copper foil having a thickness of 10 mu m was used for the negative electrode of the evaluation cell. The solid content of the negative electrode mixture prepared at the time of forming the negative electrode active material layer was set at a weight ratio of negative electrode active material: thickener: binder = 98: 1: 1. Herein, carboxymethyl cellulose (CMC) is used as a thickener. Further, styrene-butadiene rubber (SBR) is used as a binder.

&Quot; Negative electrode active material particle 710A of evaluation cell &

Graphite particles whose pitch is mixed with graphite particles serving as nuclei of the negative electrode active material particles 710A and fired and at least partially covered with the amorphous carbon film 750 are used for the negative electrode active material particles 710A of the evaluation cell 10). Herein, in each evaluation cell, the vertical direction N1 of the graphite particles 710A of the first region A1 and the verticality N2 of the graphite particles 710A of the second region A2 are different from each other, The negative electrode sheet 240A on which the active material layer 243A is formed is used. Each evaluation cell is manufactured under the same conditions except for the negative electrode sheet 240A.

In the evaluation cell, first, predetermined conditioning is performed.

«Conditioning»

Here, the conditioning is performed by the following steps 1 and 2.

Step 1: After reaching 4.1 V with constant current charge of 1C, stop for 5 minutes.

Step 2: After Step 1, charge for 1.5 hours with constant voltage charging and stop for 5 minutes.

«Measurement of rated capacity»

After the conditioning, the rated capacity is measured for the evaluation cell. The rated capacity is measured by the following steps 1 to 3. Here, the rated capacity is measured in a temperature environment of a temperature of 25 占 폚 in order to make the influence by the temperature constant.

Step 1: After reaching 3.0 V by the constant current discharge of 1C, discharge by the constant voltage discharge for 2 hours and then stopping for 10 seconds.

Step 2: After reaching 4.1 V by constant current charging of 1C, charge for 2.5 hours by constant voltage charging and then stop for 10 seconds.

Step 3: After reaching 3.0 V by a constant current discharge of 0.5 C, discharge by constant voltage discharge for 2 hours, and then stop for 10 seconds.

Rated capacity: The discharge capacity (CCCV discharge capacity) in the discharge from the constant current discharge to the constant voltage discharge in step 3 is taken as the rated capacity. In this evaluation cell, the rated capacity is approximately 1 Ah.

«SOC adjustment »

The SOC adjustment is adjusted by the following steps 1 and 2. Here, the SOC adjustment may be performed after the conditioning process and the measurement of the rated capacity. Here, SOC adjustment is performed under a temperature environment of 25 DEG C in order to make the effect of temperature constant.

Step 1: Charge with a constant current of 3V to 1C, and charge state (SOC 60%) of approximately 60% of the rated capacity.

Step 2: After Step 1, charge the battery at constant voltage for 2.5 hours.

Thereby, the evaluation cell can be adjusted to a predetermined charging state.

Then, the DC resistance, the diffusion resistance, the capacity retention rate, and the rate of increase in resistance were evaluated for these evaluation cells.

«DC Resistance»

Here, the direct current resistance is a resistance based on the electric resistance and the electrolyte resistance in the lithium ion secondary battery, and can be measured by an alternating current impedance measurement method. 14 is a diagram showing a typical example of a Cole-Cole plot (Nyquist plot) in the AC impedance measurement method. As shown in Fig. 14, the DC resistance (Rsol) and the reaction resistance (Rct) can be calculated based on the Cole-Cole plot obtained by the equivalent circuit fitting in the ac impedance measurement method. Here, the reaction resistance Rct can be obtained by the following equation.

Rct = (Rsol + Rct) -Rsol

Such measurement and calculation of the direct current resistance (Rsol) and the reaction resistance (Rct) can be carried out by using a commercially available apparatus that has been programmed in advance. As such an apparatus, there is, for example, an electrochemical impedance measuring apparatus manufactured by Solartron. Here, a complex impedance measurement is performed in a frequency range of 10-1 to 105 Hz based on an evaluation cell adjusted to an SOC of 40% (a charging state of about 40% of a rated capacity) at a room temperature (about 25 ° C) . Here, as shown in Fig. 14, the reaction resistance Rsol obtained by the equivalent circuit fitting of the Nyquist plot was defined as &quot; DC resistance &quot;.

«Diffusion resistance»

The diffusion resistance is a resistance based on the diffusion of lithium ions and is measured by the following procedure. Here, the rated capacity is measured in a temperature environment at a temperature of 25 占 폚 in order to make the influence by the temperature constant.

Step 1: The cell for evaluation is adjusted to SOC 60%, the CC discharge (constant current discharge) is performed at 1C for 45 seconds, and the voltage of the evaluation cell after discharge is measured.

Step 2: Adjust the cell for evaluation to SOC 60%, perform CC discharge (constant current discharge) at 30C for 45 seconds, and measure the voltage of the evaluation cell after discharge.

Step 3: The difference between the voltages of the evaluation cell after discharge measured in steps 1 and 2 is obtained.

«Capacity retention rate»

Here, the capacity retention ratio (capacity retention ratio after cycling) is calculated by dividing the initial capacity of the evaluation cell adjusted to a predetermined charging state and the capacity of the evaluation cell after a predetermined charge-discharge cycle (hereinafter, ) ((Post-cycle capacity) / (initial capacity)).

Quot; post-cycle capacity retention rate &quot; = (post-cycle capacity) / (initial capacity) x 100 (%

Here, the &quot; initial capacity &quot; is the discharge capacity measured based on the evaluation cell adjusted to SOC 60% at 25 ° C. Here, the &quot; discharge capacity &quot; refers to the total discharge capacity (discharge capacity) measured when discharging is performed at a constant current of 1 C from 4.1 V to 3.0 V at 25 캜 and subsequently discharged to a constant voltage until the total discharge time reaches 2 hours. to be.

The &quot; post-cycle capacity &quot; performs a predetermined charge / discharge cycle in the evaluation cell at a predetermined temperature environment. Then, based on the evaluation cell after charging and discharging, the discharge capacity measured in a temperature environment of 25 캜 is measured. The measurement of the &quot; discharge capacity &quot; here corresponds to the measurement of the &quot; discharge capacity &quot; of the initial capacity. Here, the capacity retention rate (capacity retention ratio after cycle) is the capacity retention rate after a predetermined charge / discharge cycle of 8000 cycles in a temperature environment of -30 占 폚. The capacity maintenance rate is obtained by subjecting the evaluation cell to the above-described conditioning in a temperature environment of 25 占 폚, then discharging it to 3.0 V at constant current, then charging it at a constant current constant voltage to adjust the SOC to 60%. Thereafter, a predetermined charge / discharge cycle may be performed.

Here, one cycle of charge / discharge is performed in the order of (I) to (IV) below.

(I) Discharge at a constant current of 30 C for 0.1 second (CC discharge).

(II) Discharge at a constant current of 5C for 0.4 seconds (CC discharge).

(III) Charge at a constant current of 30 C for 0.5 seconds (charge CC).

(IV) 5C, 20 seconds of CC-CV discharge, the SOC is 60%.

(I) to (IV), a predetermined interval (for example, about 10 minutes) may be provided. In addition, in the measurement of the capacity retention ratio after 8000 cycles at -30 占 폚, one cycle of charge and discharge consisting of (I) to (IV) is repeated 8,000 times.

«Resistance increase rate»

Here, the rate of increase in resistance is also referred to as a rate of resistance increase (high rate discharge deterioration rate) after a high rate discharge cycle. In this case, the above conditioning was performed on the evaluation cell at a temperature of 25 占 폚, then the cell was discharged at a constant current of 3.0 V and then charged at a constant current and a constant voltage to obtain a SOC (state of charge) of 60% 60%). Then, after the charging and discharging at the high rate is repeated, the rate of increase in resistance of the evaluation cell is measured. Here, in order to make the influence by the temperature constant, the high rate deterioration test is performed under a temperature environment of generally 20 to 25 占 폚.

One cycle of charging and discharging at the high rate is as follows (I) to (V).

(I) Discharge for 10 seconds at a constant current of 30C.

(II) Stop for 10 seconds.

(III) 5C for 60 seconds (1 minute).

(IV) Stop for 10 minutes.

The resistance of the evaluation cell with respect to the discharge of (I) is measured every (V) cycles.

One charge / discharge cycle consisting of (I) to (V) is repeated 6,000 times. At this time, the evaluation cell is adjusted to SOC 60% every 100 cycles as described above. The rate of increase of the high rate discharge resistance of the evaluation cell is calculated by multiplying the resistance (Ω ₁ ) measured in the first cycle and the resistance (Ω _E ) measured in the 6000th cycle in the charge / The resistance increase rate is calculated based on the ratio (? _E /? ₁ ).

&Quot; resistance increase rate &quot; = (? _E /? ₁ )

«Samples and their evaluation»

The present inventor has found that the negative electrode active material layer 243A having a different degree of verticalness N1 of the graphite particles 710A in the first region A1 and a vertical degree N2 of the graphite particles 710A in the second region A2, And a negative electrode sheet 240A having the negative electrode sheet 240A formed thereon. Then, the DC resistance, the diffusion resistance, the capacity retention rate, and the resistance increase rate were measured for each evaluation cell. Table 1 to Table 3 show the results of the test. 15 is a plot of data of Table 1 for each sample. In Fig. 15, the plot group D1 of the square black square "■" indicates the DC resistance, and the plot group D2 of the black rhombus "◆" indicates the diffusion resistance. 16 is a plot of data of Table 2 for each sample. 17 is a plot of data of Table 3 for each sample. In Fig. 17, the plot group D3 of the square black square "■" represents the capacity retention rate, and the plot group D4 of the black rhombus "◆" represents the resistance increase rate.

The graphite particles 710A in the first region A1 near the anode current collector 241A have a verticality N1 of N1? 1 and the graphite particles 710A in the second region A2 near the front side, The DC resistance and the diffusion resistance tend to be suppressed to be low (for example, samples 1 to 5) when the vertical degree N2 of N2 is 1.2.

When the verticality N1 of the graphite particles 710A in the first region A1 and the verticality N2 of the graphite particles 710A in the second region A2 are all in the range of 0.3 to 0.8, (For example, Samples 6 to 8). Even if the vertical degree N1 of the graphite particles 710A in the first region A1 near the anode current collector 241A is about 1.2 to 2.2, the graphite particles 710A in the second region A2 near the surface side ) Was about 0.4 to 0.5, the diffusion resistance tended to be high (for example, samples 9 and 10). When the verticality N1 of the graphite particles 710A in the first region A1 and the verticality N2 of the graphite particles 710A in the second region A2 are all about 1.3 to 1.5, (For example, Sample 11).

The graphite particles 710A in the vicinity of the anode current collector 241A have a vertical degree N1 of N1? 1 and a graphite particle 710A of the second region A2 When the degree N2 is N2? 3.0, the diffusion resistance tends to be greatly reduced (for example, Samples 12 to 14).

The difference N2-N1 between the verticality N2 of the graphite particles 710A in the second region A2 and the verticality N1 of the graphite particles 710A in the first region A1 is, for example, (N2-N1) &gt; = 1.4. In this case, the diffusion resistance tends to be low (for example, Samples 3 to 5). The (N2-N1) may be (N2-N1)? 2.0, more preferably (N2-N1)? 2.4. As a result, the diffusion resistance tends to be significantly lowered (for example, Samples 12 to 14).

If the vertical degree N1 of the graphite particles 710A in the first region A1 is N1? 1.2 and the vertical degree N2 of the graphite particles 710A in the second region A2 is N2? 0.9 . Thus, the capacity retention rate can be kept high and the rate of increase in resistance can be kept low (for example, Samples 21 to 25). In this case, the vertical degree N1 of the graphite particles 710A in the first region A1 may be N1? 1.4. In addition, the vertical degree N2 of the graphite particles 710A in the second region A2 may be N2? 0.7. In this case, the difference (N2-N1) between the perpendicularity N2 of the graphite particles 710A in the second region A2 and the perpendicularity N1 of the graphite particles 710A in the first region A1, (N2-N1)? -0.8.

When the verticality N1 of the graphite particles 710A in the first region A1 and the verticality N2 of the graphite particles 710A in the second region A2 are all about 0.3 to 0.6, (For example, Samples 26 and 27). When the verticality N1 of the graphite particles 710A in the first region A1 and the verticality N2 of the graphite particles 710A in the second region A2 are all in the range of 0.4 to 0.6, When the vertical degree N2 of the graphite particles 710A in the second region A2 near the side of the second region A2 is as high as about 0.8 to 2.0, the capacity retention rate tends to decrease (for example, Samples 28 to 30). Therefore, in order to increase the capacity retention rate and to suppress the rate of increase in resistance, the vertical degree N1 of the graphite particles 710A in the first region A1 is N1? 1.2, and the graphite particles 710A in the second region A2 And the vertical degree N2 of the particle 710A is N2? 0.9.

The lithium ion secondary battery 100A according to the embodiment of the present invention has been described above, but the lithium ion secondary battery according to the present invention is not limited to any of the embodiments described above.

INDUSTRIAL APPLICABILITY As described above, the present invention contributes to improvement of output characteristics of a lithium ion secondary battery. Therefore, the lithium ion secondary battery according to the present invention is particularly useful as a hybrid automobile having a high level required for output characteristics or cycle characteristics at high rates, a plug-in hybrid vehicle having a high level required for capacity, And is suitable for a secondary battery for a vehicle driving power source such as a driving battery for an automobile.

In this case, as shown in Fig. 18, for example, a vehicle drive battery 1000 (a motor) for driving a motor (electric motor) of a vehicle 1 such as an automobile in the form of a combined battery in which a plurality of secondary batteries are connected and combined ). Particularly, in the embodiment of the present invention, the lithium ion secondary battery can appropriately suppress the DC resistance and the diffusion resistance. Further, in one embodiment of the present invention, the capacity retention rate of the lithium ion secondary battery can be kept high, and the rate of increase in resistance can be suppressed to a low level. As a result, the lithium ion secondary battery according to the present invention is particularly suitable as a vehicle-driven battery 1000 in which the resistance is kept low, the capacity is kept high, or the rate of increase in resistance is required to be kept low. The lithium ion secondary battery according to one embodiment of the present invention is suitable for a lithium ion secondary battery having a rated capacity of 3.0 Ah or more, for example, as a battery for driving a hybrid vehicle (in particular, a plug-in hybrid vehicle) Do.

1: vehicle
100, 100A: Lithium ion secondary battery
200, 200A: wound electrode body
220: Positive electrode sheet
221: positive electrode current collector
222: Uncoated portion
223: positive electrode active material layer
224: middle portion
225: gap (cavity)
240, 240A: negative electrode sheet
241, 241A: anode collector
242, 242A: Uncoated portion
243, 243A: Negative electrode active material layer
245: Clearance (cavity)
262, 264: separator
280: electrolyte
290: Charger
300: Battery case
310, 312: clearance
320:
322: Joint of cover and container body
340: cover
350: Injection hole
352: sealing cap
360: Safety valve
420: electrode terminal
420a: the tip of the electrode terminal 420
440: Electrode terminal
440a: the tip of the electrode terminal 440
610: positive electrode active material particle
620: Conductive material
630: Binder
710: Negative electrode active material particle
710A: Graphite particles (negative electrode active material particles)
730: binder
750: Amorphous carbon film
1000: Vehicle drive battery
A1: first region
A2: second region
WL: winding axis

Claims (16)

The negative electrode collector,
And a negative electrode active material layer formed on the negative electrode collector,
Wherein the negative electrode active material layer includes scaly graphite particles and the graphite particles have different vertical degrees in a first region in the vicinity of the negative electrode current collector and a second region in the vicinity of the surface side,
Wherein the graphite particles have a vertical degree that the number of the graphite particles having a slope? N of 60 占?? N? 90 占 with respect to the surface of the negative electrode current collector is m1 and a slope? N ) Is 0 DEG &amp;le;&amp;le; n &amp;le; 30 DEG,
And the vertical degree of graphite particles = (m1 / m2). delete The method according to claim 1, wherein the slope (? N) of the graphite particles to the negative-
Sectional SEM images of a plurality of cross sections are prepared on the negative electrode active material layer formed on the negative electrode current collector,
Wherein a predetermined number of graphite particles are extracted from a cross section SEM image of a plurality of cross sections having a larger cross sectional area of an outer appearance,
And the slope of a straight line along the longest diameter in the cross section of the extracted graphite particles with respect to the surface of the negative electrode collector. 4. The method according to any one of claims 1 to 3, wherein an absolute value of a difference (N2-N1) between a perpendicularity (N1) of the graphite particles in the first region and a verticality (N2) Of 0.2 or more. The lithium secondary battery according to claim 1 or 3, wherein the graphite particles in the first region have a vertical degree (N1) of N1 &lt; = 1, and the graphite particles in the second region have a vertical degree (N2) Secondary battery. 6. The lithium ion secondary battery according to claim 5, wherein the graphite particles in the second region have a vertical degree (N2) of N2? 3.0. The method according to claim 5, wherein the difference (N2-N1) between the perpendicularity (N2) of the graphite particles in the second region and the perpendicularity (N1) , A lithium ion secondary battery. The method according to claim 5, wherein the difference (N2-N1) between the perpendicularity (N2) of the graphite particles in the second region and the perpendicularity (N1) , A lithium ion secondary battery. The lithium secondary battery according to any one of claims 1 to 3, wherein the graphite particles in the first region have a vertical degree (N1) of N1? 1.2 and the graphite particles in the second region have a verticality (N2) Secondary battery. The method according to claim 9, wherein the difference (N2-N1) between the perpendicularity (N2) of the graphite particles in the second region and the perpendicularity (N1) , &Lt; / RTI &gt; lithium ion secondary battery. The positive electrode according to claim 1 or 3, wherein the first region is a region of 0% to 30% in thickness from the negative electrode collector of the negative electrode active material layer,
And the second region is a region of a thickness of 70% to 100% from the negative electrode collector of the negative electrode active material layer. A step A of preparing a negative electrode mixture prepared by mixing at least graphite particles in the form of scales and a binder in a solvent,
And a step B of applying the negative electrode mixture prepared in the step A to the negative electrode current collector and forming a negative electrode active material layer on the negative electrode current collector,
The step B is a step of applying the negative electrode mixture to the negative electrode current collector, a drying step of drying the negative electrode material mixture applied to the negative electrode current collector, a step of applying a magnetic field to the direction of the graphite particles in the applied negative electrode mixture And an aligning step of adjusting the thickness
Wherein the coating step and the drying step are performed at least twice, and the negative electrode mixture is applied to the negative electrode collector again,
The alignment step is performed at least once before the drying step after the application step,
Wherein the alignment step is performed such that a vertical extent of the graphite particles is different between a first region in the vicinity of the negative electrode current collector and a second region in the vicinity of the surface side. The method of manufacturing a lithium ion secondary battery according to claim 12, wherein the alignment step is performed before the drying step after the last coating step, and graphite particles in the negative electrode material mixture applied in the last coating step are formed on the negative electrode current collector . 14. The method of manufacturing a lithium ion secondary battery according to claim 13, further comprising a rolling step of rolling the negative electrode mixture layer formed on the negative electrode collector before the last coating step. 13. The method according to claim 12, wherein the alignment step is performed before the first drying step after the coating step, and the graphite particles in the negative electrode mixture mixed in the first coating step are formed on the negative electrode current collector, &Lt; / RTI &gt; The method for manufacturing a lithium ion secondary battery according to claim 15, comprising a rolling step of rolling the negative electrode mixture layer formed on the negative electrode current collector after the last drying step.

Images